refactor: delegate manifest/apply/repo/workspace/gc logic to gion-core

[tasuku43]

Overview

• switch gion internals to call gion-core for planner/apply/repo/workspace/gc related logic
• keep existing CLI behavior while moving reusable logic behind gion-core APIs
• update CI to fetch local gion-core replacement dependency

Main Refactors

• manifest/apply path: planner diff, destructive checks, execution ordering, prefetch keys
• repo/store path: spec normalization, store normalization/access flow, parser helpers
• workspace/gc path: status parsing, risk classification/aggregation, gc fetch+merge checks
• import/manifest ls path: inventory reconstruction and drift/risk planning via gion-core

Dependency

• companion core PR: feat: extract reusable planning/apply/git logic into gion-core gion-core#1

Verification

• go test ./...
• go vet ./...
• go build ./...